1. Field of the Invention
The present invention relates to a light emitting module and a light receiving module for use in, for example, an optical wireless transmitter and an optical wireless receiver, respectively, of an optical wireless transmission system for transmitting information data such as a video signal, an audio signal, and a digital data signal, as an optical signal through a free space, and more particularly to a light emitting module which allows reduction of variations in brightness of an emitted light and enhancement of efficiency, and a light receiving module having enhanced light collection efficiency.
2. Description of the Background Art
An optical wireless transmission system for transmitting an optical signal through a free space has advantageous characteristics that transmission can be performed at a higher speed than radio transmission using a radio wave because a light enables a broadband transmission. In order to incorporate, in a mobile device, an optical wireless transmitter and an optical wireless receiver of the optical wireless transmission system, it is necessary to reduce the thickness and the size of each of a light emitting module and a light receiving module for use therein.
For some conventional light emitting module, a Fresnel lens is used so as to change an emission angle of a light emitted from a light source, thereby realizing reduction of the thickness thereof (see, for example, Japanese Laid-Open Patent Publication No. 2005-49367). FIG. 27 is a diagram illustrating a conventional light emitting module disclosed in Japanese Laid-Open Patent Publication No. 2005-49367.
As shown in FIG. 27, a light emitting module 10 includes a light source 1 and a lens 2. The lens 2 has, on an emission surface thereof, a plurality of refraction surfaces 3 provided so as to form concentric circles each having an optical axis at the center thereof and having diameters different from each other such that the lens 2 acts as a Fresnel lens, and therefore the lens 2 refracts a light emitted from the light source 1 so as to emit a light substantially parallel to the optical axis 4. The Fresnel lens is allowed to have a lens portion thinner than a spherical lens having a continuous curved surface, and a non-spherical lens, that is, the lens portion of the Fresnel lens is allowed to have its thickness reduced to that of a plate. In other words, the Fresnel lens has a characteristic that the thickness of the Fresnel lens can be easily reduced. However, due to, for example, limitations in processing the plurality of refraction surfaces 3 so as to make inclination angles thereof, the Fresnel lens is configured such that a reception angle 2β at which a light emitted from the light source 1 is received has only a limited value. Therefore, when an angle at which the light source 1 emits a light is large, the light emitting module 10 may not efficiently emit the light.
Further, Japanese Laid-Open Patent Publication No. 2005-49367 discloses that a range of the reception angles at which a light emitted from the light source 1 is received is increased. FIG. 28 is a diagram illustrating another conventional light emitting module 11, which is disclosed in Japanese Laid-Open Patent Publication No. 2005-49367.
As shown in FIG. 28, the light emitting module 11 includes a light source 1 and a lens 12. The lens 12 has, on an emission surface thereof, a plurality of refraction surfaces 13 which are similar to the plurality of refraction surfaces 3 of the lens 2 shown in FIG. 27, and has, on an incident surface thereof, a plurality of reflection surfaces 15 which are provided so as to form concentric circles each having an optical axis at the center thereof and having diameters different from each other. A portion of a light having been emitted from the light source 1 at an emission angle larger than 2β is reflected by the plurality of reflection surfaces 15, so as to be emitted from a plane portion 16 of the emission surface of the lens 2.
On the other hand, some conventional light receiving module for converting an incident light to an electrical signal incorporates a Fresnel lens as a collecting lens for collecting the incident light on a light receiving element, thereby reducing the thickness thereof (see, for example, Japanese Laid-Open Patent Publication No. 3-60080). FIG. 30 is a diagram illustrating a conventional light receiving module 20 as disclosed in Japanese Laid-Open Patent Publication No. 3-60080.
As shown in FIG. 30, the light receiving module 20 includes a collecting lens 21 and a light receiving element 22. The collecting lens 21 acts as a Fresnel lens having, on an incident surface thereof, a plurality of refraction surfaces 23 which are provided so as to form concentric circles each having an optical axis at the center thereof, and having diameters different from each other. The collecting lens 21 collects an incident light on the light receiving element 22. The collecting lens 21 acting as the Fresnel lens is allowed to have its thickness reduced as compared to a convex lens of a spherical surface, and the like.
FIG. 29 is an enlarged view of the A1 portion of the light emitting module 11 of the conventional configuration shown in FIG. 28. As shown in 29, the plurality of refraction surfaces 13 each has, at the tip thereof, a lens invalid portion which prevents a light emitted from the light source 1 from passing therethrough. Therefore, a dark portion preventing the light from passing therethrough occurs. Therefore, the light emitting module 11 has a problem that a light emitted from the lens 12 has variations in brightness.
Further, the conventional light emitting module 11 as shown in FIG. 28 emits a light reflected by the plurality of reflection surfaces 15, from the plane portion 16 of the emission surface, and the plane portion 16 corresponds to a portion outward from the outermost circumference of the plurality of refraction surfaces 13. Therefore, the diameter d2 of the plurality of reflection surfaces 15 needs to be larger than the diameter d1 of the plurality of refraction surfaces 13, thereby increasing the diameter of the lens 12.
Further, FIG. 31 is an enlarged view of the A2 portion of the conventional light receiving module 20 as shown in FIG. 30. As shown in FIG. 31, the plurality of refraction surfaces 23 each has, at the tip thereof, a lens invalid portion on which collection of an incident light is prevented. Therefore, the light receiving module 20 has a problem that light collecting efficiency is reduced due to an area in which no incident light is collected.